1. Field of the Invention
The present invention relates to an optical film. In particular, the present invention relates to a diffusion film applicable to a backlight module for liquid crystal displays.
2. Description of the Prior Art
Liquid crystal panels are being substituted for cathode-ray tubes and become a main stream in the market because of the merits of high definition, low irradiation, low power consumption and better utilization of space. Since liquid crystal panels cannot emit light, it is necessary to use a backlight module as a light source to offer distributed light, so that the display device can display images normally and can obtain sufficient luminance and contrast.
The main elements consisting of a backlight module include incident light, a reflective film, a light guide plate, a diffusion plate, a diffusion film, brightness enhancement film and a prism protective film and so on. The main function of the diffusion plate and diffusion film is to provide liquid crystal displays with a uniform surface light. Normally, the diffusion plate and diffusion film include two types, i.e., the inner-diffusion type and the surface-diffusion type. The inner-diffusion type is consisting of polycarbonate (PC), polystyrene or polymethyl methacrylate (PMMA) resins with organic or inorganic particles. By the utilization of diffusion particles, the light will be refracted and scattered because it passed through two media with different refractive indexes, so that the linear light emitted from the light source can be diffused into a uniform surface light. That is, the light from a below light source is scattered by the diffusion plate, uniformly dispersed upward, and then emitted. The surface-diffusion type reflects and refracts light by roughening the surface of a transparent resin plate. However, the surface-diffusion type is time-consuming and complicated in preparation procedure and is relative expensive in cost.
It is known that particles with different particle sizes can be used as diffusion particles so as to enhance the light diffusion effect. For a conventional diffusion film, a resin coating containing a plurality of diffusion particles with different particle sizes is formed on a substrate. The diffusion particles normally have a particle size in the range of 1 μm to 50 μm. The diffusion particles used in the prior art have a wide particle size distribution. That is, the particle size distribution of the diffusion particles in the resin coating (y represents the quantity of particles, while x represents the particle size) is a wide single-peak distribution. For example, when the particles used have a mean particle size of about 15 μm, the particle size distribution of the particles generally ranges from about 1 μm to about 30 μm. It is also known in the art that a mixture of more than two groups of particles with different mean particle sizes can be used as diffusion particles. That is, the particle size distribution of the diffusion particles (y represents the quantity of particles, while x represents the particle size) is a two (multi)-peak distribution. Although the light diffusion effect can be improved by using diffusion particles having a wide single-peak particle size distribution or a two (multi)-peak particle size distribution, the light will be scattered randomly due to the different particle sizes of the particles, and as a result, the light source cannot be efficiently utilized.
It is known that, if the diffusion particles in the coating are aggregated or adhered to each other, not only is the light diffusion uniformity affected, but dark spots are also likely to be generated on the surface of the display. In order to solve the above problems, U.S. Pat. No. 7,218,450 B2 discloses using one or more organic or inorganic particles with a single distribution as diffusion particles with certain parameters, including the lamination ratio, particle size of the flocculated particles, and when two kinds of particles with a single distribution are used, the mean particle sizes of the two kinds of particles with a single distribution, that meet special formulae. 95% of the particles with a single distribution used in U.S. Pat. No. 7,218,450 B2 have a particle size ranging within ±15% of the mean particle size. U.S. Pat. No. 7,218,450 B2 further teaches using diffusion particles having a narrow particle size distribution, but is silent on the crosslinking degree of the diffusion particles. In fact, an insufficient crosslinking degree of the diffusion particles will inevitably cause some problems. For example, particles with a low crosslinking degree are likely to interact with the solvent in the binder and thus swell up. Therefore, compared with the particles with a high crosslinking degree, the particles with a low crosslinking degree will have a low solvent resistance. Furthermore, as the volume of the particles with a low crosslinking degree is changed because the solvent is absorbed, the optical properties of the particles become unstable, and the viscosity on the surfaces of the particles increases, so that the particles are easily aggregated with each other, thereby further affecting the coating processibility and light diffusion effects of the diffusion film.
In addition, in various optical films, the brightness enhancement film is relatively expensive, so in the newly developed backlight module structures, modifications were made to the other optical films and the combinations thereof so as to substitute for the brightness enhancement film and to reduce the cost. For example, in liquid crystal displays, two or three diffusion films were used to replace the conventional design of the brightness enhancement film with two diffusion films respectively located on and below the brightness enhancement film. Nevertheless, the luminance and the other performances are inferior compared with the conventional design. Therefore, for current techniques, the design of the diffusion film not only focuses on meeting the light diffusion efficiency requirement, but the means of improving the luminance of the diffusion film also needs to be considered.